In recent years, laser light is used for various types of processing. Laser light with a wavelength approximately in a range from 532 nm to 1064 nm has a high energy intensity, and are suitably used for various types of processing such as cutting or welding of metal, glass, and the like. Laser light with a wavelength in a deep ultraviolet region, which is approximately from 200 nm to 350 nm, is used for processing electronic materials and composite materials.
Laser light-source apparatus that outputs laser light with a wavelength shorter than those in a near-infrared region includes: a seed light source that outputs laser light having a wavelength in the near-infrared region; an optical amplifier that amplifies the laser light output from the seed light source; and a nonlinear optical element that converts the wavelength of the laser light, amplified by the optical amplifier, into a target wavelength.
Various optical amplifiers and the like are used for various seed light sources selected to achieve a pulse width of several hundreds of picoseconds or shorter and a frequency of several hundreds of megahertz or lower, so that a laser pulse light with large peak power is obtained.
Some conventional configurations use a mode-locked laser with a pulse rate of several tens of megahertz as such a seed light source, and pulse light of several kilohertz is obtained by dividing the frequency of the pulse light output from the seed light source.
Unfortunately, the mode-locked laser involves an oscillating frequency that is fluctuated by environmental factors such as temperature and vibration and thus is difficult to appropriately control. Thus, the frequency division needs to be synchronized with the oscillating frequency of the laser pulse light detected by using a light-receiving element and the like. Thus, a complex circuit configuration is required. Furthermore, long term stable driving is difficult to achieve because the mode-locked laser includes a saturable absorber, which is apt to degrade.
Use of a semiconductor laser that emits pulse light with a controllable oscillating frequency for the seed light source might seem like a solution. Unfortunately, the semiconductor laser is only capable of emitting near-infrared pulse light with extremely small pulse energy of several picojoules to several hundreds of picojoules. Thus, to eventually obtain the pulse light with the pulse energy of several tens of microjoules to several tens of millijoules, much stronger amplification is required than in the case where the conventional seed light source is used.
Suitable examples of the optical amplifier achieving such strong amplification include: a fiber amplifier such as an erbium-doped fiber amplifier and an ytterbium-doped fiber amplifier; and a solid state amplifier such as Nd:YAG obtained by adding neodymium to yttrium aluminum garnet and Nd:YVO4 obtained by adding neodymium to yttrium vanadate.
Patent Literature 1 and Patent Literature 2 each disclose an optical amplifier as a combination of the fiber amplifier and the solid state amplifier described above. As described in Patent Literature 1 and Patent Literature 2, the fiber amplifier and the solid state amplifier both require an excitation light source for amplifying light with the same wavelength as laser light amplified by a pumping effect in a laser active region. Generally, a semiconductor laser is used for such an excitation light source.